.TH std::fopen 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::fopen \- std::fopen

.SH Synopsis
   Defined in header <cstdio>
   std::FILE* fopen( const char* filename, const char* mode );

   Opens a file indicated by filename and returns a file stream associated with that
   file. mode is used to determine the file access mode.

.SH Parameters

   filename - file name to associate the file stream to
   mode     - null-terminated character string determining file access mode

   File access flags

    File access        Meaning         Explanation      Action if file  Action if file
    mode string                                         already exists  does not exist
   "r"            read              Open a file for    read from start  return NULL and
                                    reading                             set error
   "w"            write             Create a file for  destroy contents create new
                                    writing
   "a"            append            Append to a file   write to end     create new
   "r+"           read extended     Open a file for    read from start  return NULL and
                                    read/write                          set error
   "w+"           write extended    Create a file for  destroy contents create new
                                    read/write
   "a+"           append extended   Open a file for    write to end     create new
                                    read/write
   File access mode flag "b" can optionally be specified to open a file in binary mode.
   This flag has no effect on POSIX systems, but on Windows, for example, it disables
   special handling of '\\n' and '\\x1A'.
   On the append file access modes, data is written to the end of the file regardless
   of the current position of the file position indicator.
   File access mode flag "x" can optionally be appended to "w" or "w+" specifiers. This
   flag forces the function to fail if the file exists, instead of overwriting it.
   \fI(C++17)\fP
   The behavior is undefined if the mode is not one of the strings listed above. Some
   implementations define additional supported modes (e.g. Windows).

.SH Return value

   If successful, returns a pointer to the object that controls the opened file stream,
   with both eof and error bits cleared. The stream is fully buffered unless filename
   refers to an interactive device.

   On error, returns a null pointer. POSIX requires that errno is set in this case.

.SH Notes

   The format of filename is implementation-defined, and does not necessarily refer to
   a file (e.g. it may be the console or another device accessible through filesystem
   API). On platforms that support them, filename may include absolute or relative
   filesystem path.

   For portable directory and file naming, see C++ filesystem library or
   boost.filesystem.

.SH Example


// Run this code

 #include <cstdio>
 #include <cstdlib>

 int main()
 {
     int is_ok = EXIT_FAILURE;
     FILE* fp = std::fopen("/tmp/test.txt", "w+");
     if (!fp)
     {
         std::perror("File opening failed");
         return is_ok;
     }

     int c; // Note: int, not char, required to handle EOF
     while ((c = std::fgetc(fp)) != EOF) // Standard C I/O file reading loop
         std::putchar(c);

     if (std::ferror(fp))
         std::puts("I/O error when reading");
     else if (std::feof(fp))
     {
         std::puts("End of file reached successfully");
         is_ok = EXIT_SUCCESS;
     }

     std::fclose(fp);
     return is_ok;
 }

.SH Output:

 End of file reached successfully

.SH See also

   fclose  closes a file
           \fI(function)\fP
   fflush  synchronizes an output stream with the actual file
           \fI(function)\fP
   freopen open an existing stream with a different name
           \fI(function)\fP
   C documentation for
   fopen
